1. Field of the Invention
The present invention relates to the field of catalyst supports and that of filtering agents.
More particularly it relates to the use of metallic structures of high porosity produced in refractory alloys for uses in catalysts and in filtration.
2. Description of the Prior Art
The widespread use of nickel (Ni) sponges as supports and collectors for positive electrodes of metallic nickel-cadmium and nickel hydride alkaline batteries is known. These products have reticular cellular structures with total or very widely open porosity.
The nickel sponges constitute one of the families of "three-dimensional" metallic structures (or "3D") with high porosity, which also include felt type products (interlocking of nonwoven fibres), as well as woven types. The latter are most often constituted by two woven surfaces connected by coupling, by threads which are approximately transverse to the surfaces.
The "3D" metallic structures are most often produced according to a process which respects the sequence of the following operating stages: conductive activation of a base substrate of organic material; metallization by electrolysis; burning off the organic materials (original substrate and optionally the conductive activation products); deoxidation and annealing of the metallic structure obtained.
The use of these structures for applications such as filtration or catalysts often proves to be particularly useful due to their high porosity and levels of loss of potential which can be chosen as a function of requirements.
One of the limitations to these applications relates to particular thermal or chemical constraints which can be encountered, and which means that structures produced in nickel can oxidize or corrode, in an oxidizing environment, at high temperature.
In order to overcome these drawbacks and to obtain satisfactory results, it is necessary to resort to so-called refractory alloys, one of the components of which is chromium (Cr).
The production processes for "3D" metallic structures, as has been mentioned, generally require a galvanic deposition stage which makes the production of complex alloys difficult.
To respond in part to this problem, various routes have been proposed, applied to the family of sponges:
the production of sponges of alloys, by particle sintering, PA1 case hardening of chromium, on the nickel sponges. PA1 the case hardening composition, constituted by chromium powder and a gel, must penetrate uniformly inside the porous structure. The operation is difficult to carry out in a homogeneous manner with sponges with an average pore size (between 45 and 80 PPI), impossible with structures which have a very fine pore size (sponges between 80 and 100 PPI), are highly tortuous (felts), or thick and dense (woven); PA1 at the end of the treatment cycle, the case hardening composition must be eliminated; the chromium grains have a tendency to agglomerate, and mechanical operations are then necessary to rid the sponges of the case hardening composition. PA1 by immersing in a slip containing aluminum particles in suspension, PA1 by spraying a suspension of aluminum particles, PA1 by "painting" using a lacquer or paint containing aluminum particles.
The first technique is delicate to implement and is difficult to exploit industrially due to constraints related to handling the powders and the difficulty in sintering particles of refractory alloys.
The second technique requires commercially available supports (nickel sponges) and a known technique (case hardening). Thus, DUNLOP patents mention the production of a nickel-chromium alloy sponge (Ni--Cr) based on this principle.
The case hardening of chromium (chromizing) is traditionally applied to items with a simple shape, of small to medium size. When it is implemented with supports with very complex shapes, such as metallic sponges, several difficulties are apparent:
Moreover, the penetration of the chromium thus deposited within the nickel layer is superficial, as a result of the production principle. The alloys formed by diffusion of the components provided in the forms indicated is therefore heterogeneous and this limits the thermal resistance of the structure due to the risk of oxidation of the sub-layer of metal which is an alloy with a low chromium content.
The authors of the present invention have shown that in order to overcome these various problems, it is appropriate to implement specific electrolytic deposition techniques, to deposit the chromium within the porous structure, over the totality of its developed surface, and to produce the alloy by thermal diffusion between the layers created with its various constituents.
In order, in particular, to produce refractory alloys, capable of resisting high temperatures, it is necessary to form ternary alloys such as Ni--Cr--Al, Fe--Cr--Al, or quaternary alloys, Ni--Fe--Cr--Al.
These alloys could only be produced with difficulty by codepositions. It is therefore necessary to resort to successive provisions of each of the constituents of the intended alloys.
It is known that the deposition of nickel is carried out by electrolytic route, on structures of sponge type or of felt type which have been rendered conductive (activated).
Furthermore, an activation technique for porous polymeric supports (sponges, felts or woven items) has been developed and patented which allows, under excellent conditions, for a good level of conductivity to be conferred and for various metals such as nickel, iron, copper or binary or ternary alloys of these metals to be applied by electrolytic deposition.
This technique is the activation of polymer conductors by chemical deposition (French Patent No. 95 09547, publication No. 2 737 507).
The provision of chromium can also be carried out industrially by electrolytic deposition. However, this operation is usually carried out on metal items with a relatively simple shape and with smooth surfaces. In fact, a person skilled in the art knows that chromium baths used for hard chromium plating or for decorative chromium plating have a reduced throwing power, which leads to the lack of chromium deposit or a poor quality deposit in certain areas of the item to be treated, as a function of the characteristics of the lines of electric flux.
The artifices which consist, for example, of using anodes with a special shape which will concentrate the lines of electric flux in the areas where they are little present, cannot be considered for structures with such complex shapes as "3D" products (sponges, felts or woven items).
As well as the low throwing power, which leads to a lack of chromium deposit in the areas masked vis-a-vis the anode or at the core of the porous structure, the electrolysis operating conditions lead to deposition characteristics which are not satisfactory: growths on the edges or streaks on the surface of the "3D", burnt and friable deposits, etc.